1. Field of the Invention
The present invention relates to a method and apparatus for detecting air pressure in a tire for a vehicle, and particularly to a method and apparatus for detecting air pressure in a tire using a wavelet function provided on the basis of a basic wavelet function localized in time.
2. Description of the Related Art
One type of apparatus for detecting an air pressure of a tire is proposed in Japanese Patent Unexamined Publication No. Hei. 9-126932. This apparatus has the object of continuously detecting the air pressure of a tire. It includes an oscillating electric, signal output means for outputting an oscillating electric signal which includes a vibration frequency component of a tire of a vehicle, wavelet transformation means for carrying out a wavelet transformation of the oscillating electric signal outputted from the oscillating electric signal output means using a basic wavelet function which is localized in time and expanded or contracted by a scale parameter in accordance with a shift parameter indicating a time position, and for calculating a wavelet coefficient by this wavelet transformation, resonant frequency extracting means for extracting a resonant frequency of a tire based on a mode of the wavelet coefficient calculated by the wavelet transformation means and air pressure estimating means for estimating the air pressure of a tire based on the resonant frequency extracted by the resonant frequency extracting means.
The wavelet coefficients of the wavelet transformation are obtained at a plurality of predetermined frequencies near the resonant frequency and from the modes of the wavelet coefficients. A maximum frequency is extracted for every predetermined interval of time and among these maximum frequencies, the maximum frequency the number of occurrences of which is largest in a definite period of time is extracted as the resonant frequency:
However, in the apparatus for detecting an air pressure of a tire proposed by the above publication since the resonant frequencies of tires are different for every tire (they are different if the type, maker or even the wheel cover is different), in the event that a tire is mounted which is different from the tires which were originally mounted on the new car, a shift occurs between the predetermined frequency for obtaining the wavelet coefficient by the wavelet transformation and the resonant frequency of the tire, with the result that there is a problem that the detection precision is lowered.